In the assembly of semiconductor packages, molding is one of the key manufacturing processes in which semiconductor components bonded to a substrate are encapsulated with plastic or resin material, such as epoxy molding compound (EMC), to protect them from the environment. In one example, the substrates are leadframes that have semiconductor dice comprising integrated circuits attached to them. Thereafter, the integrated circuits are electrically connected to leads on the leadframes by bonding wires. The EMC will generally encapsulate the bonding wires, part of the leads and the dice.
In some semiconductor packages like molded leadless packages or Quad-Flat No-Lead (QFN) packages, the leads are exposed at the bottom of the devices for electrically connecting the packages to external devices. FIGS. 1A and 1B are an elevation view and a back view respectively of a leadframe 10 of the prior art that has been partially molded with EMC 16. The EMC 16 should not cover certain portions of the leads 14 that need to be connected to external devices. Also, it may be preferable not to cover a die-attach pad 12 on which a semiconductor die 18 has been attached to improve heat dissipation of the package.
Referring to FIG. 1A, the leadframe 10 comprises a plurality of leads 14 surrounding a centrally located die-attach pad 12. After a semiconductor die 18 has been adhesively attached to the die-attach pad 12 and electrically connected to the leads 14 by bonding wires 22, the wire-bonded leadframe 10 is then encapsulated with EMC 16, which is shown partially molded onto the leadframe 10. Referring to FIG. 1B, the partially molded leadframe 10 is shown with the leads 14 and the die-attach pad 12 exposed on the bottom surface. The EMC 16 should not conceal the leads 14 as this will affect the wettability of solder during a solder attachment process, and result in its inability to form electrical connections to external devices.
FIG. 2 is a sectional side view of a molded leadless package 20 of the prior art. For packaging of leadless semiconductor devices, EMC 16 is molded on only one side of the leadframe substrate. The opposite side of the substrate is substantially free of EMC 16. However, if the non-molded surface of the substrate is not properly sealed, the EMC 16 may seep into the non-molded side of the substrate and the excess encapsulating material may solidify to form mold flash or bleed.
One of the methods to minimize the formation of mold flash caused by leakage of molding compound onto the non-molded surfaces is through masking the bottom surface of the leadframe 10 with an adhesive tape prior to molding. In Japanese publication patent number JP2004006693 entitled “Adhesive Sheet for Manufacturing Semiconductor Device”, a tape approach is illustrated in which a polyamide tape is adhesively attached to the bottom surface of the leadframe 10 prior to a molding process.
One of the problems with using such an adhesive tape is that it incurs additional manufacturing cost and requires additional processes, such as for attaching and detaching the tape from the leadframes 10. Moreover, the tape is not environmentally friendly, and it is not re-usable.
These additional processes often result in contamination, such as remnants of adhesive remaining on the leadframe 10, and hence cause problems such as poor solderability during later processing during package assembly. Furthermore, the additional layer of soft polyamide tape attached under the leadframes 10 will introduce compliance to the leads 14 during the wire bonding process. Thus, it may result in poor wire-bonding quality. Therefore, it would be desirable to avoid the use of adhesive tape for the manufacturing of leadless packages.
If an adhesive tape is not used and sealing of the non-molded surface cannot be achieved, it is usually necessary to remove the excess encapsulation material after the molding process. There are various apparatus and methods that have been implemented in the prior art to remove the excess encapsulation material after molding.
In a laser ablation approach illustrated in U.S. Pat. No. 6,838,637 entitled “Method and Apparatus for Deflashing of Integrated Circuit Packages”, the apparatus includes two lasers for performing deflashing. A CO2 laser is used to remove a top layer of flash and a YAG laser is then used to remove a thin layer of flash remaining after the CO2 laser deflashing. The CO2 laser deflashing followed by the YAG laser deflashing seek to remove the flash and avoid damaging the semiconductor packages.
However, the laser ablation approach is costly and time consuming. It utilizes a point attack method to remove the mold flash. Thus, the processing time for deflashing a molded leadframe 10 comprising a large number of units of leadless packages 20, would be unduly long.
In a plasma etching approach illustrated in U.S. Pat. No. 6,230,719 entitled “Apparatus for Removing Contaminants on Electronic Device”, plasma gas is supplied to a reaction chamber and the encapsulated packages are exposed to the plasma gas, thereby removing the excess molding material and other contaminants from the surfaces of the packages.
The problem with using the aforesaid plasma etching approach is that the plasma gas may react with the EMC 16 to form silica residue and charred material. Hence, after the plasma etching process, the molded leadframes 10 need to undergo another cleaning process for washing and removing the residue and charred material. Therefore, the additional cleaning process may render this approach less efficient and cost effective.
It would be advantages to avoid some of the aforesaid disadvantages of the prior art by providing a simple and cost effective apparatus for removing excess encapsulating material from an unmolded surface of a molded leadframe.